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Date: Thu, 11 Aug 2011 13:40:27 +0100
Message-ID: <CAFEAcA8m1CvqZLqK5Jk1KE9rkpsK0_D8rO9GCdFigWS9RupU7g@mail.gmail.com>
From: Peter Maydell <peter.maydell@linaro.org>
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Subject: Re: [Qemu-devel] [PATCH] tcg: Reload local variables after return
	from longjmp
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Blue Swirl <blauwirbel@gmail.com>, TeLeMan <geleman@gmail.com>, Jan Kiszka <jan.kiszka@web.de>, qemu-devel <qemu-devel@nongnu.org>, David Gilbert <david.gilbert@linaro.org>

On 11 August 2011 13:16, Paolo Bonzini <pbonzini@redhat.com> wrote:
> On 08/11/2011 01:30 PM, Peter Maydell wrote:
>> Can you give more details of what compiler/platform this was
>> a problem for? My reading of the C standard is that the compiler
>> isn't allowed to trash env across this longjmp, because it's
>> a variable of automatic scope which isn't modified between the
>> setjmp and the longjmp...
>
> longjmp can destroy any non-volatile variable (-Wclobbered warns about
> this).

"All accessible objects have values [...] as of the time the longjmp
function was called, except that the values of objects of automatic
storage duration that are local to the function containing the
invocation of the corresponding setjmp macro that do not have
volatile-qualified type and have been changed between the setjmp
invocation and longjmp call are indeterminate."
 -- C99 section 7.13.2.1 para 3.

So variables may only be destroyed if they are all of:
 * local to the function calling setjmp
 * not volatile
 * changed between setjmp and longjmp

We don't change env between the setjmp and longjmp so the compiler
should not trash it. (Indeed according to Jan in
http://lists.gnu.org/archive/html/qemu-devel/2011-07/msg00144.html
-Wclobbered doesn't complain about this code.)

-- PMM